Rapid and non-destructive prediction of methylxanthine and cocoa solid contents in dark chocolate by synchronous front-face fluorescence spectroscopy and PLSR

Abstract

To circumvent the limitation of conventional high performance liquid chromatography (HPLC) method, synchronous front-face fluorescence spectroscopy was applied for the rapid and non-destructive determination of two major methylxanthines (theobromine and caffeine) and cocoa solid contents in dark chocolate for the first time. Total synchronous fluorescence spectra at wavelength interval (Δλ) between 30–100 nm of 21 brands of commercial dark chocolate bars with labeled cocoa solids of 50–100% were directly obtained at front-face geometry. The contents of the two methylxanthines were determined by HPLC. Partial least squares regression (PLSR) models for quantification were constructed, optimized, and validated by both full (leave-one-out) cross-validation and external validation. The coefficients of determination (R2) values are larger than 0.8, and the relative errors of prediction were ca. 10%. The built models were applied to the detection of four real samples and compared with HPLC method. The combination of front-face fluorescence and PLSR provides a fast and non-invasive procedure for the quantification of theobromine and caffeine in dark chocolate, as an effective alternative for the traditional HPLC method. The further application of the proposed strategy is the rough estimation of cocoa solids in dark chocolate.